#3629 Add Jenkinsfile (#3635)
#3629 Add Jenkinsfile (#3635)

Fixes #3629 

- Adds a Jenkinsfile which is used to run builds on PRs on uberjenkins that can report back on test coverage.

This can either replace or run alongside the Drone builds, no harm in keeping both for now.

Once this is in master, it will enable all open PRs to run builds automatically. The Jenkins infrastructure is set up and waiting to read a Jenkinsfile: http://uberjenkins.sc.couchbase.com/view/All/job/_sync-gateway_github_pipeline/
couchbaselabs/sync_gateway,ceejatec/sync_gateway,couchbaselabs/sync_gateway,couchbaselabs/sync_gateway,couchbaselabs/sync_gateway,ceejatec/sync_gateway,ceejatec/sync_gateway,ceejatec/sync_gateway